The present invention relates generally to a combustion chamber, particularly to a gas turbine engine combustion chamber.
In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxide of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and all the combustion air before combustion occurs. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection. Our International patent application no. WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly.
The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends, and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has two coaxial radial flow swirlers which supply a mixture of fuel and air into a primary combustion zone. An annular secondary fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture of fuel and air into a secondary combustion zone.
One problem associated with gas turbine engines is caused by pressure fluctuations in the air, or gas, flow through the gas turbine engine. Pressure fluctuations in the air, or gas, flow through the gas turbine engine may lead to severe damage, or failure, of components if the frequency of the pressure fluctuations coincides with the natural frequency of a vibration mode of one or more of the components. These pressure fluctuations may be amplified by the combustion process and under adverse conditions a resonant frequency may achieve sufficient amplitude to cause severe damage to the combustion chamber and the gas turbine engine.
It has been found that gas turbine engines which have lean combustion are particularly susceptible to this problem. Furthermore it has been found that as gas turbine engines which have lean combustion reduce emissions to lower levels by achieving more uniform mixing of the fuel and the air, the amplitude of the resonant frequency becomes greater.
The relationship between the pressure fluctuations and the combustion process may be coupled. It may be an initial unsteadiness in the combustion process which generates the pressure fluctuations. This pressure fluctuation then causes the combustion process, or heat release from the combustion process, to become unsteady which then generates more pressure fluctuations. This process may continue until high amplitude pressure fluctuations are produced.
Accordingly the present invention seeks to provide a combustion chamber which reduces or minimises the above mentioned problem.
Accordingly the present invention provides a combustion chamber comprising a plurality of combustion zones arranged in flow series defined by at least one peripheral wall, each combustion zone having at least one fuel and air mixing duct for supplying fuel and air into the respective one of the combustion zones, each of the fuel and air mixing ducts having at least one fuel injector for supplying fuel into the respective one of the fuel and air mixing ducts, the fuel injectors in the at least one fuel and air mixing duct for at least one of the combustion zones being arranged into a plurality of circumferentially arranged sectors, fuel supply means being arranged for supplying fuel to the fuel injectors, the fuel supply means being arranged for supplying a greater amount of fuel to one or more of the circumferentially arranged sectors than the remainder of the circumferentially arranged sectors to reduce the pressure oscillations in the combustion chamber.
The combustion chamber may comprise a primary combustion zone and a secondary combustion zone downstream of the primary combustion zone.
The combustion chamber may comprise a primary combustion zone, a secondary combustion zone downstream of the primary combustion zone and a tertiary combustion zone downstream of the secondary combustion zone.
Preferably the fuel injectors in the fuel and air mixing duct supplying fuel and air into the secondary combustion zone are arranged in circumferentially arranged sectors.
The fuel injectors in the fuel and air mixing duct supplying fuel and air into the tertiary combustion zone may be arranged in circumferentially arranged sectors.
The fuel injectors in the fuel and air mixing duct supplying fuel and air into the primary combustion zone may be arranged in circumferentially arranged sectors.
The at least one fuel and air mixing duct may comprise plurality of fuel and air mixing ducts.
Preferably there may be two circumferentially arranged sectors. Preferably the two circumferentially arranged sectors are halves or extend over 180xc2x0.
Alternatively there may be three circumferentially arranged sectors. The three circumferentially arranged sectors may be thirds or extend over 120xc2x0.
Alternatively there may be four circumferentially arranged sectors. The four circumferentially arranged sectors may be quarters or extend over 90xc2x0.
Alternatively there may be six circumferentially arranged sectors. The six circumferentially arranged sectors may be sixths or extend over 60xc2x0.
Alternatively there may eight circumferentially arranged sectors. The eight circumferentially arranged sectors may be eighths or extend over 45xc2x0.
Preferably the at least one fuel and air mixing duct comprises a single annular fuel and air mixing duct.
Preferably the fuel supply means comprises a plurality of fuel manifolds and a plurality of fuel valves, each fuel manifold supplying fuel to the fuel injectors in a respective of the circumferentially arranged sectors, each fuel valve controlling the supply of fuel to a respective one of the fuel manifolds.
Preferably transducer means are acoustically coupled to the combustion chamber to detect pressure oscillations in the combustion chamber.
Preferably the transducer means is arranged to send a signal indicative of the level of the pressure oscillations in the combustion chamber to a controller, the controller being arranged to send signals to the fuel valves for supplying a greater amount of fuel to one or more of the circumferentially arranged sectors than the remainder of the circumferentially arranged sectors to reduce the pressure oscillations in the combustion chamber when the pressure oscillations are above a predetermined level and for supplying equal amounts of fuel to all of the circumferentially arranged sectors to minimise emissions when the pressure oscillations are below the predetermined level.
The present invention also provides a method of operating a combustion chamber comprising a plurality of combustion zones arranged in flow series defined by at least one peripheral wall, each combustion zone having at least one fuel and air mixing duct for supplying fuel and air into the respective one of the combustion zones, each of the fuel and air mixing ducts having at least one fuel injector for supplying fuel into the respective one of the fuel and air mixing ducts, the fuel injectors in the at least one fuel and air mixing duct for at least one of the combustion zones being arranged into a plurality of circumferentially arranged sectors, fuel supply means being arranged for supplying fuel to the fuel injectors, the method comprising supplying a greater amount of fuel to one or more of the circumferentially arranged sectors than the remainder of the circumferentially arranged sectors to reduce the pressure oscillations in the combustion chamber.
Preferably the method comprises detecting the level of the pressure oscillations in the combustion chamber, determining if the pressure oscillations are above a predetermined level, supplying a greater amount of fuel to one or more of the circumferentially arranged sectors than the remainder of the circumferentially arranged sectors to reduce the pressure oscillations in the combustion chamber when the pressure oscillations are above the predetermined level or supplying equal amounts of fuel to all of the circumferentially arranged sectors to minimise emissions when the pressure oscillations are below the predetermined level.